Try setting your idle speed between 850-900 rpm with the motor warm; this may not support cold idle (which can be fixed by richening idle mixture). I believe the problem may be that your ignition timing is advancing when the motor is warm. While Webers will work on a 912 out of the box , they require work to get properly set-up including changing venturis (come with 28's; need 30-32mm), emulsion tubes (come with F11's; need F7) and jets. There are extended discussions about Webers/Solexes on the
912 BBS: use the 'Search' function.
Peter